1. Technical Field
The present disclosure relates to carbohydrates for the purification of petrochemical compounds.
2. Description of Related Art
With the expanding global demand for petrochemical feed-stocks, the development of novel, low-cost materials that reduce the impact of chemical processing on the environment is critically important. Improving the efficiency of the refinement and separation of aromatic hydrocarbons is of particular importance, given the large volumes on which these compounds are produced. The sustained interest in metal-organic frameworks1 (MOFs) as adsorbents and sequestering agents for industrially important gases,2-4 e.g., H2, CH4, CO2 and N2, as well as for the liquid-phase separation of larger molecular compounds, which include (1) constitutional isomers,5 (2) chiral compounds,6 (3) aliphatic hydro-carbons,3b,5b,7 and (4) pharmaceuticals,8 is leading to MOFs being investigated as alternatives to zeolites9 and activated carbon10 as separation media. The improvements5-7 in separation efficiencies using MOFs over traditional size- and shape-selective materials can be attributed primarily to (i) the physiochemical properties imbedded in their diverse building blocks, (ii) their higher surface areas, and (iii) their larger adsorption capacities, which reduce the amount of adsorbent required for industrial processes.7a,11 Consequently, MOFs represent emergent materials for separation technologies in many different industrial settings.
In the chemical industry, one of the most challenging separations is that of BTEX (that is, benzene, toluene, ethylbenzene, and the three regioisomers of xylene) obtained from the refining of crude oil. The xylene isomers, together with ethylbenzene, constitute the C8 aromatics that are derived12 from crude oil by catalytic reforming, toluene disproportionation, and the distillation of pyrolysis gasoline. These C8 aromatics not only act12b as octane and antiknocking additives in gasoline, but they are also important chemical feedstocks, thus bringing about the necessity for their processing and separation. The difficulty in separating p-xylene from the BTEX mixture can be ascribed to the similar physical properties of these C8 aromatics. Industrial practices12,13 focus on separation by adsorption strategies or crystallization procedures, with 60% of p-xylene produced today relying on simulated moving bed (SMB) technologies.12,13 Here, C8 aromatics are separated based on differences in adsorbate-adsorbent interactions within faujasite-type zeolites. The xylene adsorption equilibrium can be tuned by ion-exchange within the zeolite to attain9a,12a,14 p-xylene purities of approximately 95 wt % per pass. Crystallization techniques account for the purification of the remaining 40% of p-xylene produced.13b, c,15 
These energy-intensive processes highlight the need for further improvements in the technologies currently available, especially in relation to materials that can discriminate among BTEX molecules. A wide variety of materials have been investigated for the separation of aromatic hydrocarbons, such as zeolites,9,12a discrete metal complexes,16 and organic cages.17 MOFs have exhibited varying degrees of success in separating xylenes from mixtures of C8 aromatics, e.g., classical rigid MOFs, such as copper benzenetricarboxylate [Cu3(btc)2], have been employed to separate BTEX mixtures chromotographically,18 while MOF-5 shows little to no separation of the xylene isomers.19 The most widely investigated MOFs for separating aromatic hydro-carbons are the terephthalate-based structures with one-dimensional channels,20-24 namely MIL-47 and MIL-53. Both MOFs exhibit high o-xylene selectivity, separating the xylene regioisomers based on molecular packing and entropic differences.21-23,25 More recently,26,27 MIL-125 and MAF-X8 have exhibited high p-xylene affinity due to pore morphology and commensurate stacking, respectively. The guest-driven restructuring of a flexible cerium tetradentate carboxylate MOF led to high selectivity by restructuring of the framework around p- and m-xylene, displaying molecular-level recognition,28 and adding to the growing number of flexible MOFs having potential utility for separations.6h, 21,28 